Q: if a muslim girl marries a non beliver and her parents are helpless in front of her….what should be the attitude of people towards her parents?

A: If parents raised the child with good religious values, they have nothing to worry about, nor should they be helpless. People (outsiders) should not concern themselves with faults of others. We need to clean our own homes before we point towards the ills of others.
We should not be judgmental, we should advocate sympathy and give positive support to anyone in pain without patronizing an already adverse situation.

Allah certainly Knows Best.